Steel Material, Process of Fabricating Steel Material, and Apparatus of Fabricating Steel Material

ABSTRACT

According to one aspect of the present invention, there is provided a process of fabricating a steel material by performing a heat treatment to a steel material having high strength, in order to reduce hardness at one part of the steel material to less than hardness at other parts of the steel material, wherein the heat treatment comprises a heating step in which a portion having a certain depth from a surface of the steel material is rapidly heated by induction heating or direct energization heating, and a cooling step in which the steel material, which has been subject to the heating step, is rapidly cooled a predetermined time after the heating step, and a heating temperature in the heating step is Ac1 transforming point or more.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a Continuation Application of PCT Application No. PCT/JP2009/056733, filed Mar. 31, 2009, which was published under PCT Article 21(2) in Japanese.

This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2008-093760, filed Mar. 31, 2008, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a steel material, a process of fabricating the steel material, and an apparatus of fabricating the steel material, and more particularly to a steel material having different hardness depending upon a portion.

2. Description of the Related Art

There has been proposed, for example, a technique of changing a tensile strength (hardness) depending upon a portion from, for example, a surface layer to a central part, in order to enhance a delayed fracture resistance, in a secondary processing (heat treatment) for a steel material such as a coil-like rolled material (hereinafter referred to as a rolled material) that is a raw material of a rod or a wire.

For example, there has been known a technique in which a ultralow-carbon steel such as pure iron is arranged on a surface layer, the resultant is rolled, and the resultant is subject to decarburization and decrease of carbon, like a clad steel (e.g., refer to Jpn. Pat. Appln. KOKAI Publication No. 6-57367).

There have also been proposed a technique of utilizing a decarburized layer that is generated on a surface layer through an application of heat at a temperature of Ac1 to Ac3 during rolling (e.g., refer to Jpn. Pat. Appln. KOKAI Publication No. 62-267420), and a technique of performing a heat treatment again only to the surface layer after the heat treatment (e.g., refer to Jpn. Pat. Appln. KOKAI Publication No. 7-54441).

As a method of enhancing the delayed fracture resistance, there has also been known a technique of performing a chemical surface treatment of plating or nitriding, or a technique of applying a non-metallic coating material excellent in delayed fracture resistance on the surface.

BRIEF SUMMARY OF THE INVENTION

However, the above-mentioned techniques have the problems described below. Specifically, they need to perform a preprocessing, since a clad steel is fabricated at the stage of a rolled material, or since a secondary process (heat treatment) is performed after a decarburized layer is formed through the application of heat at a temperature of Ac1 to Ac3. Further, it is necessary to perform the heat treatment or the chemical surface treatment only to the surface layer after the secondary process (heat treatment). Therefore, these processes are complicated, and a complicated process condition has to be controlled before and after the secondary process (heat treatment).

In view of this, an object of the present invention is to provide a steel material having a different hardness depending upon a portion, a process of fabricating the steel material, and an apparatus of fabricating the steel material with a simple process.

According to one aspect of the present invention, there is provided a process of fabricating a steel material by performing a heat treatment to a steel material having high strength, in order to reduce hardness at one part of the steel material to less than hardness at other parts of the steel material, wherein the heat treatment comprises a heating step for rapidly heating a portion from a surface of the steel material to a certain depth by induction heating or direct energization heating, and a cooling step for rapidly cooling the steel material subjected to the heating step after a predetermined time from the heating step, and a heating temperature in the heating step is Ac1 transforming point or more.

According to another aspect of the invention, a time from the heating step to the cooling step is not more than a predetermined time that is determined according to a steel grade, a wire diameter, a heating temperature, and a heating time.

According to another aspect of the invention, a process condition in the heat treatment is determined based upon a heat conductivity characteristic of the steel material after the rapid heating to the surface.

According to another aspect of the invention, the process condition in the heat treatment is a time integration value of the temperature of the steel material, and is determined based upon a tempering progression value that indicates a progression state of tempering of the steel material.

According to another aspect of the invention, the process condition in the heat treatment is a combination including at least two of a frequency, an input electric energy, a heating temperature, a heating time, and a natural cooling time.

According to another aspect of the invention, the process further comprises a step of calculating the heat conductivity characteristic of the steel material or the tempering progression value, wherein the process condition in the heat treatment is determined based upon the calculated heat conductivity characteristic or the tempering progression value.

According to another aspect of the invention, the process condition in the heat treatment is set such that the tempering progression value at a surface layer becomes 1.5 times or more the tempering progression value at a central part.

According to another aspect of the invention, the time from the heating step to the cooling step is set such that the tempering progression value at the surface layer becomes 1.5 times or more the tempering progression value at the central part.

According to another aspect of the invention, the steel material is a steel wire or a steel rod.

According to another aspect of the invention, after a quenching process including a heating process and a cooling process is performed to the steel material, the heating step and the cooling step are respectively performed once as the tempering process.

According to another aspect of the invention, there is provided a steel material that is subject to the heating step and the cooling step, wherein a difference between hardness in the vicinity of a surface layer and hardness at a position toward a central part from a position of 10% from the surface layer in a radial direction is HV50 or more, and a tensile strength obtained when a tensile test is conducted with a No. 2 test piece in JISZ2201 is 1420 N/mm² or more.

According to another aspect of the invention, all cross-sections have a tempered martensite structure, hardness of a surface layer is HV380 or less, a tensile strength obtained when a tensile test is conducted with a No. 2 test piece in JISZ2201 is 1420 N/mm² or more, and hardness at a portion near a central part from the surface layer is uniform.

According to another aspect of the invention, all cross-sections have a tempered martensite structure, hardness of a surface layer is HV420 or less, a tensile strength obtained when a tensile test is conducted with a No. 2 test piece in JISZ2201 is 1600 N/mm² or more, and hardness at a portion near a central part from the surface layer is uniform.

According to another aspect of the invention, there is provided an apparatus of fabricating a steel material that performs a heat treatment to a steel material having high strength, in order to reduce hardness at one part of the steel material to less than hardness at other parts of the steel material, the apparatus comprising: a heating unit configured to rapidly heat a portion from a surface of the steel material to a certain depth by induction heating or direct energization heating; and a cooling unit configured to rapidly cool the steel material subjected to the heating after a predetermined time from the heating, wherein a heating temperature of the steel material by the heating unit is Ac1 transforming point or more.

According to another aspect of the invention, the apparatus further comprises a control unit configured to control the process condition in the heat treatment based upon a calculation result of the heat conductivity characteristic of the steel material or the tempering progression value.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is an explanatory view schematically illustrating a configuration of a heat treatment apparatus according to a first embodiment of the present invention.

FIG. 2 is an explanatory view schematically illustrating a heat treatment process according to the first embodiment.

FIG. 3 is a side view illustrating a configuration of a PC steel rod according to the first embodiment.

FIG. 4 is a table illustrating a composition of the PC steel rod according to the first embodiment.

FIG. 5 is a graph illustrating the relationship between the number of turns of a coil and a wire diameter in the heat treatment apparatus according to the first embodiment.

FIG. 6 is a table illustrating a heat treatment condition according to the first embodiment and a conventional example.

FIG. 7 is a graph illustrating a relationship between an elapsed time and a temperature distribution according to a heat-transfer analysis of the PC steel rod according to the first embodiment.

FIG. 8 is a graph illustrating a relationship between a natural cooling time and a temperature distribution according to the heat-transfer analysis of the PC steel rod according to the first embodiment.

FIG. 9 is a graph illustrating a relationship between hardness and a distance in a diameter direction of the PC steel rod according to the first embodiment.

FIG. 10 is a graph illustrating a temperature distribution from a surface layer to a central part during a tempering process according to the first embodiment.

FIG. 11 is a graph illustrating an efficiency of N parameter in the first embodiment.

FIG. 12 is a graph illustrating a relationship between a ratio of the N parameter at the surface and the central part and the heating time and the natural cooling time according to the first embodiment.

FIG. 13 is a graph illustrating a distribution of a cross-section hardness of the PC steel rod obtained through the heat treatment according to the first embodiment and through the conventional heat treatment.

FIG. 14 is a graph illustrating a distribution of the cross-section hardness in the axial direction of the PC steel rod fabricated by the heat treatment according to the first embodiment.

FIG. 15 is a table illustrating the composition of plural types of the PC steel rods used for the heat treatment according to the first embodiment.

FIG. 16 is a table illustrating the result of a delayed fracture resistance test of a PC steel rod W according to the heat treatment in the first embodiment and the conventional heat treatment.

FIG. 17 is a graph illustrating the result of the delayed fracture resistance test.

FIG. 18 is a side view illustrating a configuration of a PC steel rod W1 according to the first embodiment.

FIG. 19 is a side view illustrating a configuration of a notch of the PC steel rod W1 according to the first embodiment.

FIG. 20 is a table illustrating the result of the delayed fracture resistance test of the PC steel rod obtained through the heat treatment according to the first embodiment and through the conventional heat treatment.

FIG. 21 is a graph illustrating the result of the delayed fracture resistance test.

FIG. 22 is a graph illustrating the relationship between the depth of the notch formed on the PC steel rod W1 and a fracture time according to the first embodiment.

FIG. 23 is an explanatory view schematically illustrating a configuration of a heat treatment apparatus according to another embodiment of the present invention.

FIG. 24 is a side view illustrating a configuration of a deformed PC steel rod according to a second embodiment of the present invention.

FIG. 25 is a table illustrating a composition of the deformed PC steel rod according to the second embodiment.

FIG. 26 is a table illustrating a heat treatment condition in the heat treatment according to the second embodiment and a comparative heat treatment.

FIG. 27 is a graph illustrating a simulation result of an elapsed time and a temperature change in the heat treatment of the deformed PC steel rod by a heat-transfer analysis according to the second embodiment.

FIG. 28 is a perspective view illustrating a configuration of a spring steel wire according to a third embodiment of the present invention.

FIG. 29 is a table illustrating a composition of the spring steel wire according to the third embodiment.

FIG. 30 is a table illustrating a heat treatment condition in the heat treatment according to the third embodiment and a comparative heat treatment.

FIG. 31 is a graph illustrating a simulation result of an elapsed time and a temperature change in the heat treatment by a heat-transfer analysis according to the third embodiment.

FIG. 32 is a graph illustrating a distribution of a cross-section hardness of the spring steel wire after the treatment in the heat treatment according to the third embodiment and the comparative heat treatment.

FIG. 33 is a graph illustrating the relationship between the hardness and a fatigue limit of a heat-treated material.

FIG. 34 is a table illustrating a result of a rotational bending fatigue test of a comparative heat-treated material.

FIG. 35 is a graph illustrating a relationship between a distance from a surface layer to an inclusion and the number of times of durability of the comparative heat-treated material.

FIG. 36 is a table illustrating a result of a rotational bending fatigue test of the spring steel wire by the heat treatment according to the third embodiment.

FIG. 37 is a graph illustrating a cross-section hardness, residual stress, and stress amplitude of the spring steel wire in the heat treatment according to the third embodiment and the comparative heat-treated material.

FIG. 38 is a table illustrating an example of a composition of a spring steel wire according to another embodiment.

FIG. 39 is a perspective view illustrating a configuration of a bolt according to a fourth embodiment of the present invention.

FIG. 40 is a table illustrating a composition of the bolt according to the fourth embodiment.

FIG. 41 is a table illustrating heat treatment conditions in the heat treatment according to the fourth embodiment and a comparative heat treatment.

FIG. 42 is a graph illustrating a simulation result of an elapsed time and a temperature change in the heat treatment of a bolt Wb by a heat-transfer analysis according to the fourth embodiment.

FIG. 43 is a graph illustrating a distribution of the cross-section hardness of the bolt obtained through the heat treatment according to the fourth embodiment and a bolt obtained through the comparative heat treatment.

FIG. 44 is a table illustrating the result of a delayed fracture resistance test of the bolt obtained through the heat treatment according to the fourth embodiment and the bolt obtained through the comparative heat treatment.

FIG. 45 is a graph illustrating an accumulated fracture probability and a fracture time of the bolt obtained through the heat treatment according to the fourth embodiment and the bolt obtained through the comparative heat treatment.

DETAILED DESCRIPTION OF THE INVENTION

A first embodiment of the present invention will be described with reference to FIGS. 1 to 22. FIG. 1 is a conceptual view of a heat treatment apparatus 10 according to the present embodiment. FIG. 2 is a flowchart illustrating a process of fabricating a PC steel rod according to the present embodiment.

As illustrated in FIG. 1, the heat treatment apparatus 10, which is one example of an apparatus of fabricating a steel material, comprises a pinch roller 11 (conveying means) that conveys a PC steel rod W, which is one example of a steel material, a quenching heating coil 12 and a quenching cooling jacket 13, which are quenching means, a pinch roller 14 (conveying means), a tempering heating coil 15 that serves as heating means for performing high-frequency induction heating, a cooling jacket 16 serving as cooling means, and a pinch roller 17 (conveying means), which are arranged along a linear conveying path. The heat treatment apparatus 10 also has a function of heating and cooling the PC steel rod (steel) W while conveying the same along the conveying path.

The PC steel rod W, which is a subject to be processed, is a solid round bar as illustrated in FIG. 3. It is continuously conveyed in the axial direction. The PC steel rod W includes compositions illustrated in FIG. 4, but it is not limited thereto.

The tempering heating coil 15 has a function of performing high-frequency induction heating on the PC steel rod W passing therethrough. The tempering heating coil 15 is set to have a suitable number of turns according to a wire diameter or a conveying speed. In the present embodiment, the number of turns of the tempering heating coil 15 is 6, for example. However, the invention is not limited thereto. As a comparison, a conventional apparatus using a coil having 17 turns is illustrated. FIG. 5 illustrates the relationship among the general number of turns of the coil, the wire diameter, and the conveying speed.

During the tempering with the high-frequency induction heating, the PC steel rod W itself generates heat. The depth of the heat-generating part can be adjusted by the combination of the number of turns and the frequency of the tempering heating coil 15, input electric energy, heating temperature, heating time, and natural cooling time.

The cooling jacket 16 has a function of injecting cooling liquid to the passing PC steel rod W in order to cool the rod.

The distance from the tempering heating coil 15 to the cooling jacket 16 is set to be 500 mm or less, for example. In the ordinary apparatus, this distance is about 1900 mm. In the present embodiment, the distance is set to be short in order to shorten the time from the heating process to the cooling process.

FIG. 6 illustrates one example of a process condition of the heat treatment according to the present embodiment. The process condition in FIG. 6 has been found by the present applicant by utilizing the temporal change in a heating temperature pattern at the moment the high-frequency induction heating is performed, and the tempering characteristic of the steel, from the principle described later. When the heat treatment is continuously performed under this process condition, a steel rod W having a layer with low hardness as a surface layer, a uniform hardness distribution from a certain depth, and a strength level of 1420 N/mm² or more can be fabricated with one tempering.

The process condition in the present embodiment is set such that the frequency is 50 kHz, the quenching heating temperature is 1000° C., the tempering heating temperature is 805° C., the tempering heating time is 0.17 s, and the time taken from the tempering heating to the cooling is 0.63 s. The PC steel rod used here is a small-diameter PC steel rod having a diameter d (nominal designation) of 7.1 mm. The tempering heating temperature is adjusted such that the tensile strength becomes about 1440 N/mm².

The process condition for the conventional steel material, which is a comparative example, is set such that the frequency is 9.5 kHz, the quenching heating temperature is 1000° C., the tempering heating temperature is 603° C., the tempering heating time is 0.59 s, and the time taken from the tempering heating to the cooling is 3.48 s. The conventional steel material is also a small-diameter PC steel rod having a diameter d (nominal designation) of 7.1 mm. The tempering heating temperature is adjusted such that the average tensile strength on all cross sections becomes about 1440 N/mm². The composition of the steel rod in the present embodiment and the composition of the conventional steel rod are the same.

Specifically, the tempering heating temperature in the present embodiment is higher than that for the conventional product, and the time from the tempering heating to the cooling is set to be shorter than the time for the conventional product.

As for the tempering temperature, heating with the tempering temperature not less than Ac1 transforming point (727° C.) is impossible from a conventional common practice, since the heated material is quenched. In the present embodiment, it has been found that the heated material is not quenched by performing both a rapid heating and a rapid cooing to the surface, which is achieved by controlling the rapid cooling immediately after the completion of the heating, with the use of the high-frequency induction heating, even if the temperature is not less than the Ac1 transforming point.

In the heat treatment apparatus 10 according to the present embodiment, the conveying speed by the pinch roller 11, the heating temperature by the tempering heating coil 15, the heating time, and the distance between the tempering heating coil 15 and the cooling jacket 16 are appropriately set and adjusted, whereby the PC steel rod W having a desired hardness distribution can be obtained.

The operation of the heat treatment apparatus 10 thus configured will be described with reference to the flowchart in FIG. 2. A continuous wire rod W0, which is a linear or bar-like continuous steel material that is subject to a drawing process in a material process, is continuously conveyed by the pinch roller 11 (conveying means) from left to right in FIG. 1. The conveyed continuous wire rod W0 is rapidly heated to a quenching temperature with induction heating by the quenching heating coil 12 during a quenching process, and then, rapidly cooled by injecting quenching cooling liquid from the quenching cooling jacket 13 in order to be continuously quenched.

The continuous wire rod W0 to which the quenching process is performed is heated while passing through the tempering heating coil 15. The continuous wire rod W0, which has been heated to a predetermined tempering temperature, is conveyed to the cooling jacket 16 that continuously ejects a cooling liquid, so that the continuous wire rod W0 is rapidly cooled by the cooling jacket 16. The continuous wire rod W0 passes through the cooling jacket 16, whereby the total length thereof is cooled, and then, the tempering heating process is completed. The W0 is carried out by the pinch roller 17. After the heat treatment is completed, the continuous wire rod W0 (PC steel rod) is subject to a processing process and checking process, whereby it is made as a product (PC steel rod W).

The principle for determining the process condition in the heat treatment will next be described.

An N parameter value (tempering progression value), which is a new parameter serving as a reference for setting the process condition, will be described later.

FIG. 7 illustrates a simulation result according to a heat-transfer analysis using a finite element model (FEM) of an elapsed time from the start of the heating to a certain section of a steel rod, which is currently and continuously heated, to 0.8 s, and a temperature change. For 6 parts from the surface to the center, the abscissa represents elapsed time, while the ordinate represents temperature. The heating time is set to be 0.17 s here. Therefore, the first (left in the graph) 0.17 s up to 0.8 s is a heating time, and the remaining 0.63 s at the right in the graph is a natural cooling time.

As a general tempering parameter, Larson-Miller parameter P=T×(A+logt) [T: temperature (K), A: constant, t: time (h)], which is established for the case of a long-time heating, has been known. As the parameter P increases, the tempering progresses more (hardness is low).

On the other hand, when a rapid-heating process and a rapid-cooling process are both established in the high-frequency induction heating illustrated in FIG. 7, for example, as a short-time heating in the high-frequency induction heating, a certain N value can be set as a parameter. The defined N value (N parameter value) is a time integration of the temperature of the PC steel rod W, and represented by the following equation (1):

N=∫ ₀ ^(t0) T(t)dt  (1)

In the equation 1, T is a temperature (° C.), t is a time (s), and t0 is a heat treatment time (s).

Specifically, the N parameter value is an area enclosed below each curve in the graph. As the N parameter value increases, the tempering progresses more (hardness is low).

In FIG. 7, when t0 is 0.8, i.e., at the point of 0.8 s from the start of heating and when the natural cooling time is 0.63 s, the N parameter at the surface layer is greater than the N parameter value at the central part as a result of the comparison of the N parameter value represented by the area, and at the depth of about 2 mm or more, the difference between the N parameter values is not so changed. Therefore, if the steel rod is cooled at this point, the state shown in FIG. 9 in which the surface layer has low hardness and the portion from the low-hardness-layer end part to the central part has a substantially uniform hardness distribution can be realized. In the present embodiment, t0 is set to 0.8 s. However, the invention is not limited thereto. A suitable value can be set according to various conditions such as the wire diameter or the steel grade.

FIG. 8 illustrates a graph indicating the relationship between the natural cooling time and the temperature distribution according to the heat-transfer analysis of the PC steel rod using FEM. Curves in which the abscissa represents r (distance from the center)/R (radius) and the ordinate represents temperature are illustrated for every 0.3 s. The analysis condition is such that the shape is a solid round bar, the radius is 3.65 mm, the material is S40C, the depth of the heat-generating layer is 0.154 mm, the heating time is 0.17 s, the natural cooling time is 0.63 s, and the initial temperature is 20° C. t in the graph represents the time, corresponding to FIG. 7, from the start of heating, and indicates the analysis result from 0.17 s when the natural cooling is started to 0.8 s when the natural cooling is completed. Therefore, t=0.8 s corresponds to the natural cooling time of 0.63 s.

As illustrated in FIG. 8, the heat caused by the heat generation on the surface layer is transmitted to the center of the PC steel rod or to the outside with time. At the initial heating, the temperature distribution in which the surface layer has high temperature and the central part has low temperature appears. With the lapse of time, the temperature at the surface layer and the temperature at the central part become uniform. At the point of t=0.8 s, the temperature Ts at the surface layer is 430° C., and the temperature at the central part is 429.1° C.

FIG. 9 illustrates a graph in which a cross-section hardness of the PC steel rod W is measured. The abscissa represents position in the diameter direction, while the ordinate represents Vickers hardness. The hardness corresponds to the tensile strength. The state in which the surface layer has low hardness and the portion from the low-hardness-layer end part to the central part has a substantially uniform hardness can be realized.

According to FIG. 8, a soaking state is established after 0.63 s. Therefore, it is found that, as the time up to the cooling increases, the difference in the N parameter values between the surface layer and the central part is eliminated.

FIG. 10 illustrates the N parameter value and the Vickers hardness after 0.8 s from the start of heating of the PC steel rod W. There is a symmetric relationship between the N parameter value and the Vickers hardness, and they agree well with each other, so that the hardness can be arranged with the N parameter.

Organizing the principle described above, since the tensile strength of the PC steel rod W is determined by the N parameter value that is the progression state of the tempering from the surface to the center, it is understood that there is a certain range of tensile strength in order to satisfy the standard. When the temperature distribution and the time up to the cooling are controlled within this range in order to increase the difference between the N parameter at the surface layer and the N parameter at the central part as much as possible, a steel rod having a desired property can be obtained.

Specifically, a process of fabricating a steel material having a layer with low hardness as a surface layer, a uniform hardness distribution from a certain depth, and a tensile strength, e.g., a strength level of 1420 N/mm² or more, can be realized with one tempering in a continuous heat treatment by utilizing a temporal change in the heating temperature pattern at the moment of the high-frequency induction heating and the tempering property of the steel material.

It has generally been known that the delayed fracture resistance is excellent as the tensile strength is low. Specifically, the PC steel rod W having a low-hardness portion on the surface has both an excellent delayed fracture resistance and a predetermined tensile strength. The PC steel rod W described above can be fabricated according to the process described above.

Specifically, during the high-frequency tempering, the depth of the heat-generating portion can be adjusted by selecting a suitable coil, frequency, input electric energy, heating temperature, heating time, and natural cooling time, whereby the pattern calculated by the simulation can be realized. Accordingly, the time up to the cooling is adjusted during the high-frequency heating, resulting in that the tensile strength of the whole PC steel rod satisfies the standard, but only the surface layer can be made to have low hardness. In the external heating of a radiation system, other than the high-frequency heating, such as a furnace heating, the temperature rise and the soaking state are not achieved in a short-period process, but a steel material continues to be slowly heated. Therefore, the difference in strength in the radius direction cannot be produced from the viewpoint of the tempering property of the steel material. On the other hand, in the high-frequency heating, the heating is performed in a short period such as 1 s or less, so that the internal hardness other than the surface layer can be made uniform.

Based on the N parameter, and by utilizing the temperature distribution (difference between the N parameters) that is specific to the high-frequency heat treatment, a continuous high-frequency heat treatment line that can adjust the hardness distribution on the surface layer and can allow the tensile strength to satisfy the standard value for the steel rod with one tempering can be realized.

For example, when the N parameter at the surface layer is set to be 1.5 times or more the N parameter at the central part, a desired satisfactory softening at the surface layer can be achieved.

The N parameter can preferably be used for a steel material of 100 kg/mm² degree or more. A steel of a common steel having C of 0.1 mass % to 0.5 mass % is preferable in terms of the limitation on the tempering temperature. The preferable range of the diameter by which the N parameter acts as the principle is, for example, 5 to 40 mm.

Specifically, the N parameter utilizes the tempering of the high-strength steel, and utilizes an overshoot due to the rapid heating and the rapid transfer to the uniform heating by the heat conductivity of the steel. Therefore, when the diameter is larger than the above-mentioned range, it is difficult to perform the tempering in order to make the whole steel have uniform hardness within the standard strength within the range (the N parameter at the surface layer is about 1.5 times or more the N parameter at the central part) by which the desired softening of the surface layer can be achieved. Note that the present invention is not limited thereto. The present invention is applicable to a steel rod having a large diameter exceeding the above-mentioned range, from the viewpoint of the high-frequency tempering.

This also indicates the time constraint. Specifically, as illustrated in FIG. 11 that shows the efficiency of the N parameter, it is found that the cooling is preferably performed in a short period considering the state in which the whole steel rod is tempered, e.g., in 0.8 s or less in FIG. 8, in the continuous heating line. Specifically, since the overshooting time is very short, the tempering progresses with Larson-Miller parameter as is well known, when the N parameter value at the surface layer is not 1.5 times or more the N parameter value at the central part. Therefore, in an industrial continuous heat treatment, high strength cannot be secured when 10 s or more is taken for the cooling. Accordingly, the time constraint for the N parameter is produced.

FIG. 11 illustrates the relationship between the N parameter and the Vickers hardness. The N parameter value and the hardness are determined by the equation in FIG. 11. Therefore, if a suitable coil, frequency, input electric energy, heating temperature, heating time, and natural cooling time are selected, and the N parameter value is estimated by the simulation, a target hardness can be obtained.

FIG. 12 illustrates the ratio of the N parameters at the central part and the surface layer, and the elapsed time from the start of heating. At the point of around 1 s from the start of heating, the state in which the N parameter at the surface layer is 1.5 times or more the N parameter at the central part can be realized, whereby only the surface layer has low hardness. The PC steel rod W obtained by the above-mentioned heat treatment has a tempered martensite structure for all cross-sections.

FIG. 13 illustrates the distribution of the cross-section hardness of the PC steel rod W obtained by the above-mentioned heat treatment. In FIG. 13, the abscissa represents distance from the surface layer, while the ordinate represents Vickers hardness. As illustrated in FIG. 13, the distribution of the cross-section hardness in an ordinary PC steel rod is uniform, but it can be confirmed that the PC steel rod according to the present invention has low hardness in the vicinity of the surface layer and uniform hardness at the central part.

FIG. 14 illustrates the distribution of the cross-section hardness, in the axial direction, of the PC steel rod fabricated by the heat treatment according to the present embodiment. The cross-section hardness at 6 parts, each of which has a different distance from the surface layer, is shown. In FIG. 14, the ordinate represents Vickers hardness, while the abscissa represents distance in the axial direction from a reference point. It can be confirmed from FIG. 14 that the hardness distribution of the PC steel rod W is substantially constant in the axial direction.

FIG. 16 shows a result of a delayed fracture resistance test of two types of PC steel rods as the PC steel rod W, which use two types of steel grades having different compositions illustrated in A and B in FIG. 15, when the heat treatment is performed by the heat treatment process under the condition shown in FIG. 6. The steel grades A, B, and C are round bar PC steel rod, while the steel grade D is a deformed PC steel rod in FIG. 15. FIG. 17 illustrates the result of the test in FIG. 16 in the form of a graph. The ordinate represents fracture time, and the abscissa represents accumulated fracture probability. FIG. 17 indicates that, the longer the fracture time, the more excellent the delayed fracture resistance.

The delayed fracture resistance test was conducted in such a manner that each steel rod was immersed in 20% NH₄SCN solution whose temperature was kept at 50° C., and with this state, a load of 1420×0.7 N/mm² was applied thereto.

It can be confirmed from FIG. 17 that various types of the steel rods to which the heat treatment according to the present embodiment is performed are excellent in the delayed fracture resistance compared to the steel rod to which an ordinary heat treatment is performed.

FIG. 17 shows that the steel rod plotted at the upper part as a whole (i.e., the fracture time is long) is excellent in the delayed fracture resistance. Specifically, it can be understood from FIG. 17 that the PC steel rod processed under the condition according to the present embodiment is excellent in the delayed fracture resistance, compared to the conventional one.

As for the steel grade that is not fractured even by the delayed fracture test, a sample provided with a notch 20 was formed, and the delayed fracture test was carried out. FIGS. 18 and 19 illustrate the configuration of the PC steel rod W1 provided with the notch 20.

FIG. 20 shows a result of a delayed fracture test of two types of PC steel rods as the PC steel rod W, which use two types of steel grades having different compositions illustrated in C and D in FIG. 15, when the heat treatment is performed by the heat treatment process under the condition shown in FIG. 6. FIG. 21 illustrates the accumulated fracture probability.

The delayed fracture test was conducted in such a manner that each steel rod was immersed in 20% NH₄SCN solution whose temperature was kept at 50° C., and with this state, a load of 1420×0.8 N/mm² was applied thereto. FIG. 21 illustrates the result in FIG. 20 as a graph in which the ordinate represents fracture time and the abscissa represents accumulated fracture probability. FIG. 21 shows that, the longer the fracture time, the more excellent the delayed fracture resistance.

FIG. 22 illustrates the relationship between the depth of the notch 20 formed on the W1 and the fracture time. It can be confirmed that even the PC steel rod W1 provided with the notch 20 is excellent in the delayed fracture resistance, when it is formed by the heat treatment according to the present embodiment, compared to the steel rod formed by the ordinary heat treatment. In FIG. 22, the ordinate represents average fracture time, and the abscissa represents depth of the notch 20. The effect of enhancing the delayed fracture resistance according to the present embodiment is sharply reduced from the depth of 0.4 mm. Since the diameter of the PC steel rod W1, which is used this time, is 7.2 mm, the effect by the present embodiment can be confirmed within 10% of a sample radius from the surface layer, i.e., about 0.36 mm from the surface layer.

The effect described below can be obtained according to the PC steel rod, the heat treatment process of the PC steel rod, and the heat treatment apparatus according to the present embodiment.

The present invention can provide a PC steel rod having excellent delayed fracture resistance with a simple process by combining the surface heating caused by the high-frequency induction heating and the tempering property of the steel. Specifically, the present invention utilizes the temporal change in the temperature pattern at the moment of heating and the tempering property, whereby a steel rod having a different hardness depending upon a portion can be obtained with a simple process at one tempering that satisfies the predetermined process condition.

The present invention has found the condition under which a surface layer that is sufficiently soft can be formed and a temperature of 720° C. or more, which cannot be applied because a steel material is generally quenched by this temperature, can be applied, thanks to the high-frequency induction heating. According to the quenching and tempering heat treatment in which a steel material is rapidly heated in a short period by the high frequency, high strength and high toughness can be obtained compared to an ordinary heat treatment with a furnace heating.

Since the heat treatment condition based upon the tempering property and the heat conductivity property is found out by using the simulation result as described above, an appropriate condition that can be applied to various steel materials can easily be found out.

The present invention is not limited to the above-mentioned embodiment, and the invention can be embodied by modifying the components without departing from the scope of the invention. For example, the specific process condition can suitably be changed according to the steel grade of the subject steel material, the obtained strength standard or distribution of hardness, and the specification of the apparatus. The set process condition of the heat treatment is not limited to those described above.

The heat treatment apparatus may be configured, like a heat treatment apparatus 101 illustrated in FIG. 23, to include a detecting unit 21 that detects various pieces of information about a subject steel rod, a calculating unit 22 that calculates the simulation result illustrated in FIGS. 7 and 8 by using the finite element method, a control unit 23 such as a CPU for controlling to adjust various conditions of the apparatus 101 according to the simulation result, and an adjusting unit 24 that adjusts various settings according to the control by the control unit 23, in addition to the components in the heat treatment apparatus 10 in the first embodiment, wherein the heat treatment apparatus 101 is configured to be capable of controlling and adjusting the heat treatment condition based upon the information of the corresponding steel material. In this case, the speed of the conveying mechanism of the pinch rollers 11, 14, and 17 or the position of the cooling jacket 16 is adjusted, whereby the time from the tempering to the cooling can be controlled. The apparatus may also be configured such that the other heat treatment condition such as the heating temperature or the frequency can be controlled by the control unit 23. Also in this case, the same effect as that in the first embodiment can be obtained. The information about the steel material may be input by a user.

Second Embodiment

A second embodiment of the present invention will be described below with reference to FIGS. 24 to 27. The second embodiment is the same as the first embodiment except that the steel material, which is the subject to be processed, is a deformed PC steel rod Wc. Therefore, the overlapped description will not be repeated.

A heat treatment according to the present embodiment is referred to as a surface-layer softening process, a steel material (here, the deformed PC steel rod Wc) subject to the surface-layer softening process is referred to as a surface-layer softened material, a heat treatment as a comparative example that is a subject to be compared is referred to as a comparative heat treatment, and a deformed PC steel rod formed by the comparative heat treatment is referred to as a comparative heat-treated material.

In the present embodiment, the deformed PC steel rod Wc having a spiral groove formed continuously and uniformly on a surface layer as illustrated in FIG. 24 is used as a steel material to be processed. The heat treatment apparatus 10 illustrated in FIG. 1 is used, and the process according to the fabrication process illustrated in FIG. 2 is applied, as in the first embodiment. Specifically, the present embodiment is different from the first embodiment only in the shape, composition, diameter, etc. of the steel material that is the subject to the processed. Therefore, the heat treatment condition is determined by the same principle as that in the first embodiment and the same parameter as that in the first embodiment.

The present embodiment illustrates the case of using the deformed PC steel rod Wc containing the composition illustrated in FIG. 25, but the invention is not limited thereto.

FIG. 26 shows the heat treatment condition for the deformed PC steel rod Wc according to the present embodiment and the heat treatment condition in the comparative heat treatment that is the subject to be compared. The heat treatment condition for the deformed PC steel rod Wc in the present embodiment (surface-layer softened material) is set such that the frequency is 50 kHz, the quenching heating temperature is 1000° C., the tempering heating temperature is 805° C., tempering heating time is 0.17 s, and the time from the tempering heating to the cooling is 0.63 s. The deformed PC steel rod Wc used here has a diameter db (nominal designation) of 7.1 mm, wherein the tensile strength for all cross-sections is adjusted to be about 1400 N/mm².

The comparative process condition is set such that the frequency is 9.5 kHz, the quenching heating temperature is 1000° C., the tempering heating temperature is 603° C., tempering heating time is 0.59 s, and the time from the tempering heating to the cooling is 3.48 s. The comparative heat-treated material is also a deformed PC steel rod having a diameter of 7.1 mm, wherein the average tensile strength for all cross-sections is adjusted to be about 1400 N/mm². The composition of the comparative heat-treated material is the same as the composition of the deformed PC steel rod Wc that is the surface-layer softened material in the present embodiment.

FIG. 27 illustrates the simulation result of the relationship between the elapsed time and the temperature change of the heat treatment for the deformed PC steel rod Wc according to the present embodiment. FIG. 27 illustrates the relationship between the elapsed time and the temperature according to the distance from the surface.

It is understood from FIG. 27 that the surface layer part has a greater area of a hatched section in the figure than the central part. Specifically, the surface layer part is kept at high temperature for a long period, so that the hardness is greatly reduced.

The present embodiment also provides the same effect as that in the first embodiment. Specifically, the present embodiment can provide a deformed PC steel rod having excellent delayed fracture resistance with a simple process by combining the surface heating caused by the high-frequency induction heating and the tempering property of the steel. Specifically, the present invention utilizes the temporal change in the temperature pattern at the moment of heating and the tempering property, whereby a deformed PC steel rod having a different hardness depending upon a portion can be obtained with a simple process at one tempering that satisfies the predetermined process condition. Further, according to the quenching and tempering heat treatment in which a steel material is rapidly heated in a short period by the high frequency, high strength and high toughness can be obtained compared to an ordinary heat treatment with a furnace heating.

Third Embodiment

A third embodiment of the present invention will be described below with reference to FIGS. 28 to 37. The third embodiment is the same as the first embodiment except that the steel material, which is the subject to be processed, is a spring steel wire Ws. Therefore, the overlapped description will not be repeated.

A heat treatment according to the present embodiment is referred to as a surface-layer softening process, a steel material (here, the spring steel wire Ws) subject to the surface-layer softening process is referred to as a surface-layer softened material, a heat treatment as a comparative example is referred to as a comparative heat treatment, and a spring steel wire formed by the comparative heat treatment is referred to as a comparative heat-treated material.

The steel material that is the subject to be processed is the spring steel wire Ws illustrated in FIG. 28 in the present embodiment. The heat treatment apparatus 10 illustrated in FIG. 1 is used, and the process according to the fabrication process illustrated in FIG. 2 is applied, as in the first embodiment. Specifically, the present embodiment is different from the first embodiment only in the composition, diameter, etc. of the steel material that is the subject to the processed. Therefore, the heat treatment condition is determined by the same principle as that in the first embodiment and the same parameter as that in the first embodiment.

The present embodiment illustrates the case of using the spring steel wire Ws containing the composition illustrated in FIG. 29 as the subject to be processed, but the invention is not limited thereto.

FIG. 30 shows the heat treatment condition in the heat treatment according to the present embodiment and the heat treatment condition in the comparative heat treatment as the comparative example. The heat treatment condition of the surface layer-softening process in the present embodiment is set such that the frequency is 50 kHz, the quenching heating temperature is 950° C., the tempering heating temperature is 789° C., tempering heating time is 0.4 s, and the time from the tempering heating to the cooling is 2.6 s.

The spring steel wire Ws used here has a diameter ds of 12.0 mm, wherein the tensile strength for all cross-sections is adjusted to be about 1900 N/mm².

The comparative heat treatment condition in the comparative example is set such that the frequency is 9.5 kHz, the quenching heating temperature is 950° C., the tempering heating temperature is 495° C., tempering heating time is 1.7 s, and the time from the tempering heating to the cooling is 11.1 s.

The comparative heat-treated material is also a spring steel wire having a diameter of 12.0 mm, wherein the average tensile strength for all cross-sections is adjusted to be about 1900 N/mm². The composition of the comparative heat-treated material is the same as the composition of the spring steel wire Ws according to the present embodiment.

FIG. 31 illustrates the simulation result of the relationship between the elapsed time and the temperature change of the heat treatment for the spring steel wire Ws according to the present embodiment. FIG. 31 illustrates the relationship between the elapsed time and the temperature according to the distance from the surface.

It is understood from FIG. 31 that the surface layer part has a greater area of a hatched section in the figure than the central part. Specifically, the surface layer part is kept at high temperature for a long period, so that the hardness is greatly reduced.

FIG. 32 illustrates the distribution of the distance from the surface layer and the hardness. In FIG. 32, the ordinate represents hardness [HV0.3], while the abscissa represents distance [mm] from the surface layer. FIG. 32 illustrates the distribution of the hardness of the spring steel wire (comparative heat-treated material) to which the heat treatment as the comparative example is performed and the distribution of the hardness of the spring steel wire Ws (surface-layer softened material) to which the process is performed under the heat treatment condition according to the present embodiment.

In the comparative heat-treated material, the hardness hardly changes even if the distance from the surface layer changes. On the other hand, in the spring steel wire Ws to which the heat treatment according to the present embodiment is performed, it is found that the hardness is changed such that the hardness in the vicinity of the surface layer increases as the distance from the surface layer increases. Specifically, Hv<500 is established within the range of 1 mm from the surface layer.

As illustrated in FIG. 33, in the case of hardness [Hv]>500, an early breakage from an inclusion is generally likely to occur when a fatigue test is conducted. Therefore, in the spring steel wire Ws (surface-layer softened material) in which the surface layer is softened through the process under the heat treatment condition according to the present embodiment, a fatigue property can be enhanced.

FIG. 34 illustrates the relationship between the distance from the surface layer to the inclusion and the number of times of durability as a result of a rotational bending fatigue test for the comparative heat-treated material as the comparative example. FIG. 35 is a graph illustrating the relationship between the distance from the surface layer to the inclusion and the number of times of durability of the comparative heat-treated material. It can be confirmed from FIGS. 34 and 35 that, as the distance from the surface layer to the inclusion increases, the number of times of durability tends to increase in the comparative heat-treated material.

FIG. 36 illustrates the relationship between the distance from the surface layer to the inclusion and the number of times of durability as a result of a rotational bending fatigue test for the spring steel wire Ws (surface-softened material) processed according to the present embodiment.

As for the result of the rotational bending fatigue test, the case in which the rotational bending fatigue test is executed after a shot peening process is illustrated. The condition of the test is set such that the stress amplitude is 700 MPa, and the rotational speed is 2000 rpm. The test was conducted up to ten million times. In the graph, “>1000” means that the wire is not broken even after ten million times.

Comparing FIGS. 34 and 36, the number of times of durability is remarkably increased in the spring steel wire Ws (surface-layer softened material) according to the present embodiment compared to that of the spring steel wire (comparative heat-treated material) in the comparative example, which means the fatigue property is enhanced. It is confirmed that the breakage is always produced from the surface layer in the spring steel wire Ws (surface-layer softened material) according to the present embodiment.

FIG. 37 is a graph illustrating the distribution of the cross-section hardness, residual stress, and stress amplitude. The abscissa represents distance [mm] from the surface layer, while the ordinate represents hardness [HV0.3], the stress amplitude [MPa], and the residual stress [MPa].

The distribution of the residual stress of the spring steel wire Ws and the distribution of the residual stress of the comparative heat-treated material are similar to each other. The maximum compression stress appears in the vicinity of 0.1 mm from the surface layer, and then, the tensile stress appears at 0.2 mm or more from the surface layer.

As can be understood from the graph, since the comparative heat-treated material has high hardness, and low toughness in the case of the same stress amplitude, the early breakage is produced from the inclusion at the portion of 0.2 to 1.0 mm from the surface layer where the residual stress is tensile stress.

On the other hand, in the spring steel wire Ws that is the surface-layer softened material according to the present embodiment, the surface layer has low hardness but high toughness. Therefore, the formation of the fatigue crack from the inclusion can be suppressed even at the portion near the surface layer, i.e., at the portion of 0.2 to 1.0 mm from the surface layer. As a result, the number of times of durability is significantly improved.

The present embodiment also provides the same effect as that in the first embodiment. Specifically, the present embodiment can provide a spring steel wire having excellent delayed fracture resistance with a simple process by combining the surface heating caused by the high-frequency induction heating and the tempering property of the steel. Specifically, the present invention utilizes the temporal change in the temperature pattern at the moment of heating and the tempering property, whereby a spring steel wire having a different hardness depending upon a portion can be obtained with a simple process at one tempering that satisfies the predetermined process condition. Further, according to the quenching and tempering heat treatment in which a steel material is rapidly heated in a short period by the high frequency, high strength and high toughness can be obtained compared to an ordinary heat treatment with a furnace heating.

In the spring steel wire Ws fabricated according to the present embodiment, the portion within the range of 0.2 to 1.0 mm from the surface layer where the residual stress is tensile stress is softened. Therefore, the present embodiment provides an effect that the early breakage from the inclusion is hardly produced.

In the present embodiment, the composition illustrated in FIG. 29 is shown as one example. However, the present invention is not limited thereto. As another example, there are considered a steel grade E to a steel grade I containing composition illustrated in FIG. 38. In these steel materials, the vicinity of the surface layer can be softened according to the above-mentioned process, whereby high strength and high toughness can be obtained, and the early breakage from the inclusion can be prevented. The lowermost column in the table of FIG. 38 illustrates the respective compositions contained in the illustrated plural spring steel wires in the form of the range.

Fourth Embodiment

A fourth embodiment of the present invention will be described below with reference to FIGS. 39 to 45. The fourth embodiment is the same as the first embodiment except that the steel material, which is the subject to be processed, is a bolt Wb. Therefore, the overlapped description will not be repeated.

A heat treatment according to the present embodiment is referred to as a surface-layer softening process, a bolt Wb formed by the surface-layer softening process is referred to as a surface-layer softened material, a heat treatment as a comparative example is referred to as a comparative heat treatment, and a bolt formed by the comparative heat treatment is referred to as a comparative heat-treated material.

The bolt Wb illustrated in FIG. 39 is used as the subject to be processed in the present embodiment. The fabrication apparatus illustrated in FIG. 1 is used, and the process according to the fabrication process illustrated in FIG. 2 is applied, as in the first embodiment. Specifically, the present embodiment is different from the first embodiment only in the shape, composition, diameter, etc. of the steel material that is the subject to the processed. Therefore, the heat treatment condition is determined by the same principle as that in the first embodiment and the same parameter as that in the first embodiment.

The present embodiment illustrates the case of using the bolt Wb containing the composition illustrated in FIG. 40 as the subject to be processed, but the invention is not limited thereto.

FIG. 41 shows the heat treatment condition for the bolt Wb according to the present embodiment and the heat treatment condition for the bolt in the comparative heat treatment. The heat treatment condition in the present embodiment (surface layer-softened material) is set such that the frequency is 50 kHz, the quenching heating temperature is 1000° C., the tempering heating temperature is 780° C., tempering heating time is 0.15 s, and the time from the tempering heating to the cooling is 0.61 s.

The bolt Wb used here has a diameter db of 7.1 mm, wherein the tensile strength for all cross-sections is adjusted to be about 1600 N/mm².

The condition of the comparative heat treatment is set such that the frequency is 9.5 kHz, the quenching heating temperature is 1000° C., the tempering heating temperature is 480° C., tempering heating time is 0.61 s, and the time from the tempering heating to the cooling is 3.50 s. The comparative heat-treated material used here is also a bolt having a diameter of 7.1 mm, wherein the average tensile strength for all cross-sections is adjusted to be about 1600 N/mm². The composition of the bolt Wb serving as the surface-layer softened material in the present embodiment and the composition of the comparative heat-treated material are the same. The heat treatment in the present embodiment and the heat treatment in the comparative example are performed before a screw is rolled to a bar member Wb1 of the bolt.

FIG. 42 illustrates the simulation result of the relationship between the elapsed time and the temperature change of the heat treatment for the bolt Wb according to the present embodiment. FIG. 42 illustrates the relationship between the elapsed time and the temperature according to the distance from the surface.

It is understood from FIG. 42 that the surface layer part has a greater area of a hatched section in the figure than the central part. Specifically, the surface layer part is kept at high temperature for a long period, so that the hardness is greatly reduced.

FIG. 43 illustrates the distribution of the hardness of the bolt (comparative heat-treated material) processed under the comparative heat treatment condition in the comparative example and the distribution of the hardness of the bolt Wb (surface-layer softened material) processed under the heat treatment condition according to the present embodiment. In FIG. 43, the ordinate represents hardness [HV0.3], while the abscissa represents distance [mm] from the surface layer.

In the comparative heat-treated material, the hardness hardly changes even if the distance from the surface layer changes in FIG. 43. On the other hand, in the bolt Wb (surface-layer softened material) to which the heat treatment according to the present embodiment is performed, it is found that the hardness is changed such that the hardness in the vicinity of the surface layer increases as the distance from the surface layer increases. Specifically, Hv<500 is established within the range of 1.0 mm from the surface layer.

The delayed fracture resistance of the bolt Wb (surface-layer softened material) processed under the heat treatment condition according to the present embodiment is enhanced more than that of the comparative heat-treated material.

FIG. 44 illustrates a table in which the results of the delayed fracture resistance test for the bolt Wb (surface-layer softened material) according to the present embodiment and for the comparative heat-treated material are compared. The delayed fracture resistance test was carried out after a screw was rolled to the total length of the bar material Wb1 of the bolt Wb. The depth of the screw was 0.7 mm. The condition for the delayed fracture resistance test was set such that the subject was immersed into 20% NH₄SCN solution at a test temperature of 50° C., and with this state, a tensile force of 1530 N/mm²×0.7 of the tensile strength at the screw part was applied. The test was conducted until 200 hours elapsed by employing a load carrying method or a constant strain method. In the figure, “>200” means that the bolt is not broken even after the lapse of 200 hours.

FIG. 45 is a graph illustrating the relationship between the accumulated fracture probability and the fracture time of the surface-layer softened material according to the present embodiment and the comparative heat-treated material. The abscissa represents accumulated fracture probability [%], while the ordinate represents fracture time [h].

It is understood from FIGS. 44 and 45 that the comparative heat-treated material is fractured in 30 to 130 hours, while no samples of the surface-layer softened material are fractured even after the lapse of 200 hours or more. Accordingly, the delayed fracture resistance property can be enhanced by performing the surface-layer softening process.

The present embodiment also provides the same effect as that in the first embodiment. Specifically, the present embodiment can provide a bolt having excellent delayed fracture resistance with a simple process by combining the surface heating caused by the high-frequency induction heating and the tempering property of the steel. Specifically, the present invention utilizes the temporal change in the temperature pattern at the moment of heating and the tempering property, whereby a bolt having a different hardness depending upon a portion can be obtained with a simple process at one tempering that satisfies the predetermined process condition. Further, according to the quenching and tempering heat treatment in which steel is rapidly heated in a short period by the high frequency, high strength and high toughness can be obtained compared to an ordinary heat treatment with a furnace heating.

In the present embodiment, the composition illustrated in FIG. 40 is shown as one example. However, the present invention is not limited thereto.

The present invention is not limited to the above-mentioned embodiments, and the invention can be embodied by modifying the components without departing from the scope of the invention. For example, the specific process condition can suitably be changed according to the shape, steel grade, or composition of the subject steel material, the obtained strength standard or distribution of hardness, and the specification of the apparatus. The set process condition of the heat treatment is not limited to those described above.

As one example, the PC steel rod, the deformed PC steel rod, the spring steel wire, and the bolt have been illustrated in the first to fourth embodiments, and the similar result can be obtained in these embodiments by the process using the similar principle. The concept of the present invention is not limited to the illustrated steel grade, but can be applied to various other steel materials.

It has been described in the respective embodiments that the surface layer is softened by performing the tempering process to the steel material that has high strength due to the quenching process. In addition to the case described above, a steel material that has high strength due to a wire drawing, severe plastic deformation, or a carburizing process may be used.

Various inventions can be made by appropriately combining the plural components described in the respective embodiments. For example, some components may be deleted from all components described in the embodiments. The components in the different embodiments may appropriately be combined.

The present invention can provide a steel material having a different hardness depending upon a portion, a process of fabricating the steel material, and an apparatus of fabricating the steel material with a simple process. 

1. A process of fabricating a steel material by performing a heat treatment to a steel material having high strength, in order to reduce hardness at one part of the steel material to less than hardness at other parts of the steel material, wherein the heat treatment comprises a heating step for rapidly heating a portion from a surface of the steel material to a certain depth by induction heating or direct energization heating, and a cooling step for rapidly cooling the steel material subjected to the heating step after a predetermined time from the heating step, and a heating temperature in the heating step is Ac1 transforming point or more.
 2. The process of fabricating a steel material according to claim 1, wherein a time from the heating step to the cooling step is not more than a predetermined time determined according to a steel grade, a wire diameter, a heating temperature, and a heating time.
 3. The process of fabricating a steel material according to claim 1, wherein a process condition in the heat treatment is determined based upon a heat conductivity characteristic of the steel material after the rapid heating to the surface.
 4. The process of fabricating a steel material according to claim 1, wherein the process condition in the heat treatment is a time integration value of the temperature of the steel material, and is determined based upon a tempering progression value that indicates a progression state of tempering of the steel material.
 5. The process of fabricating a steel material according to claim 4, further comprising a step of calculating the heat conductivity characteristic of the steel material or the tempering progression value, wherein the process condition in the heat treatment is determined based upon the calculated heat conductivity characteristic or the tempering progression value.
 6. The process of fabricating a steel material according to claim 4, wherein the process condition in the heat treatment is set such that the tempering progression value at a surface layer becomes 1.5 times or more the tempering progression value at a central part.
 7. The process of fabricating a steel material according to claim 4, wherein the time from the heating step to the cooling step is set such that the tempering progression value at the surface layer becomes 1.5 times or more the tempering progression value at the central part.
 8. The process of fabricating a steel material according to claim 1, wherein the process condition in the heat treatment is a combination including at least two of a frequency, an input electric energy, a heating temperature, a heating time, and a natural cooling time.
 9. The process of fabricating a steel material according to claim 8, further comprising a step of calculating the heat conductivity characteristic of the steel material or the tempering progression value, wherein the process condition in the heat treatment is determined based upon the calculated heat conductivity characteristic or the tempering progression value.
 10. The process of fabricating a steel material according to claim 8, wherein the time from the heating step to the cooling step is set such that the tempering progression value at the surface layer becomes 1.5 times or more the tempering progression value at the central part.
 11. The process of fabricating a steel material according to claim 1, wherein the steel material is a steel wire or a steel rod.
 12. The process of fabricating a steel material according to claim 1, wherein, after a quenching process including a heating process and a cooling process is performed to the steel material, the heating step and the cooling step are respectively performed once as the tempering process.
 13. A steel material that is subject to the heating step and the cooling step according to claim 1, wherein a difference between hardness in the vicinity of a surface layer and hardness at a position toward a central part from a position of 10% from the surface layer in a radial direction is HV50 or more, and a tensile strength obtained when a tensile test is conducted with a No. 2 test piece in JISZ2201 is 1420 N/mm² or more.
 14. A steel material that is subject to the heating step and the cooling step according to claim 1, wherein all cross-sections have a tempered martensite structure, hardness of a surface layer is HV380 or less, a tensile strength obtained when a tensile test is conducted with a No. 2 test piece in JISZ2201 is 1420 N/mm² or more, and hardness at a portion near a central part from the surface layer is uniform.
 15. A steel material that is subject to the heating step and the cooling step according to claim 1, wherein all cross-sections have a tempered martensite structure, hardness of a surface layer is HV420 or less, a tensile strength obtained when a tensile test is conducted with a No. 2 test piece in JISZ2201 is 1600 N/mm² or more, and hardness at a portion near a central part from the surface layer is uniform.
 16. An apparatus of fabricating a steel material that performs a heat treatment to a steel material having high strength, in order to reduce hardness at one part of the steel material to less than hardness at other parts of the steel material, the apparatus comprising: a heating unit configured to rapidly heat a portion from a surface of the steel material to a certain depth by induction heating or direct energization heating; and a cooling unit configured to rapidly cool the steel material subjected to the heating after a predetermined time from the heating, wherein a heating temperature of the steel material by the heating unit is Ac1 transforming point or more.
 17. The apparatus of fabricating a steel material according to claim 16, wherein a time from an end of the process by the heating unit to a start of the process by the cooling unit is not more than a predetermined time determined according to a steel grade, a wire diameter, a heating temperature, and a heating time, a process condition in the heat treatment is determined based upon a combination including at least two of a heat conductivity characteristic of the steel material after the rapid heating to the surface, a tempering progression value indicating a progression state of tempering of the steel material, a frequency, an input electric energy, a heating temperature, a heating time, and a natural cooling time, the process condition in the heat treatment is set such that the tempering progression value at the surface layer becomes 1.5 times or more the tempering progression value at a central part, a time from the heating step to the cooling step is not more than a predetermined time determined according to a steel grade, a wire diameter, a heating temperature, and a heating time, the steel material is a steel wire or a steel rod, the apparatus further comprises: a conveying unit configured to continuously convey the steel material along a predetermined path passing the heating unit and the cooling unit, wherein the process condition includes a distance and a conveying speed in the conveying path; and a quenching unit configured to perform a heating process and a cooling process to the steel material, one heating unit and one cooling unit are provided at a downstream side of the quenching unit in the conveying path, and the apparatus further comprises a control unit configured to control the process condition in the heat treatment based upon a calculation result of the heat conductivity characteristic of the steel material or the tempering progression value. 